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Posted By: Nitin Bajaj - Editor Member Level: Diamond Posted Date: 14 Apr 2008
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2007 Jawaharlal Nehru Technological University B.Tech ELECTRICAL ENGINEERING Question paper
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II B.Tech I Semester Regular Examinations, November 2007 ELECTRICAL ENGINEERING ( Common to Mechanical Engineering, Chemical Engineering, Mechatronics, Metallurgy & Material Technology, Production Engineering and Automobile Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ?????
1. (a) Explain about “Dot convention” in the case of mutually coupled circuits. (b) Explain brie?y “How you can use mutual inductance principle to the transformer”. [8+8]
2. (a) Derive an expression for the instantaneous value of alternating current in terms of its max.value, angular velocity, and time. (b) An alternating current is represented by the following equation i = 100 Sin (100 p t) How long will it take for the current to attain values of 20,30, 50, 80 and 100A? (c) Explain the terms: frequency, cycle, and period time in connection with a.c. Circuits. [6+5+5]
3. (a) A four pole dc generator runs at 900rpm and is lap wound and has a useful ?ux per pole of 0.07wb. The armature windings consists of 220turns each of 0.004 ohms resistance. Calculate the terminal voltage when running at 900rpm if the armature current is 50A. (b) A four pole lap wound generator has 56 coils with 6 turns per coil. The speed is 1150 rpm. What must be the ?ux per pole in order to generate 265V ? How many commutator bars are required for this generator? [10+6]
4. (a) Explain DC motor principle and its working (b) A 250v shunt motor on no-load runs at 1000rpm and takes 5A the total armature and shunt ?eld resistances are 0.2 ? and 250 ? respectively calculate the speed when loaded and taking current of 50A if armature reaction weakens the ?led by 3%. [6+10]
5. (a) Derive the EMF equation of a transformer. (b) In no load test on a 1-phase transformer the following test data were obtained. i. ii. iii. iv. v. Primary voltage 220V Secondary voltage 110 V Primary current 0.5A Power input 30Watts Resistance of primary winding = 0.6? Calculate i. ii. iii. iv. turns ratio magnetizing component of no load current. working component of no load current. iron loss.
6. Explain about Delta/Delta connection of Transformers in detail A500 -KVA, 3phase 50 Hgs Transformer has a voltage Ratio (line voltages) of 33/11 KV and is Delta/Star connected. The resistances per phase are : High voltage 35?, low voltage 876 ? and the Iron loss is 3050 W. Calculate the e?ciency at full load and one-half of full load respectively (a) at unity PF (b) 0.8 PF.
7. (a) What are the three voltage drops occurring in an alternator on-load? (b) A 550 V, 55 KV A, single phase alternator has an e?ective resistance of 0.2 ohm. A ?eld current of lOA produces an armature current of 200 A on short circuit, and an emf of 450 V on open circuit. Calculate i. The synchronous reactance; ii. The full- load regulation with power factor 0.8 lagging. [8+8] [16]
8. (a) Sketch and describe the construction of a moving coil ammeter and give the principle of operation. (b) A moving coil instrument gives full scale de?ection with 15mA and has a resistance of 5?. Calculate the resistance of the necessary components in order that the instrument may be used as i. a 2A - Ammeter ii. a 100V voltmeter.
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